Because it has been doing pretty well lately, economically speaking. People go where jobs are, and Denver is one of those places these days.

As to why that is... this is still a matter of debate, but here's one take. http://washingtonmonthly.com/magazine/novdec-2015/why-denver...

In my experience (of people from the east coast), a lot of people want to move to Boulder/Denver because they're "young" cities (and pot is legal).

Strange. It's an outlier on the map, and contradicts the author's main point. Yet there's no mention of Colorado in the article.